Evidence for a mass dependent forward-backward asymmetry in top quark pair production

We present a new measurement of the inclusive forward-backward tt¯ production asymmetry and its rapidity and mass dependence. The measurements are performed with data corresponding to an integrated luminosity of 5.3fb^-1 of pp¯ collisions at s=1.96TeV, recorded with the CDF-II Detector at the Fermilab Tevatron. Significant inclusive asymmetries are observed in both the laboratory frame and the tt¯ rest frame, and in both cases are found to be consistent with CP conservation under interchange of t and t¯. In the tt¯ rest frame, the asymmetry is observed to increase with the tt¯ rapidity difference, Δy, and with the invariant mass M_tt¯ of the tt¯ system. Fully corrected parton-level asymmetries are derived in two regions of each variable, and the asymmetry is found to be most significant at large Δy and M_tt¯. For M_tt¯≥450GeV/c², the parton-level asymmetry in the tt¯ rest frame is A^tt¯=0.475±0.114 compared to a next-to-leading order QCD prediction of 0.088±0.013.